In many cases, an electromechanical power transmission chain is a parallel power transmission chain where an electric machine is mechanically connected to a combustion engine and also to one or more actuators to be driven. An actuator can be for example a wheel, a chain track, a hydraulic pump, or another device to be driven with mechanical power. The electromechanical power transmission chain comprises an energy-storage for storing electric energy and converter equipment for transferring electric energy between the energy-storage and the electric machine. The electric machine operates sometimes as a generator which charges the energy-storage, and sometimes as an electric motor which receives electric energy from the energy-storage and assists the combustion engine when high mechanical output power is needed.
An inherent challenge related to parallel power transmission chains of the kind described above is the need to control the combustion engine and the electric machine so that mechanical load constituted by one or more actuators being driven is shared dynamically in an appropriate way between the combustion engine and the electric machine. Different approaches for mechanical load sharing have been studied and published. For example, the publication Fu, Z., Gao, A., Wang, X., and Song, X.: Torque Split Strategy for Parallel Hybrid Electric Vehicles with an Integrated Starter Generator, Discrete Dynamics in Nature and Society Volume 2014, Article ID 793864, describes mechanical load sharing implemented with a fuzzy logic controller. The fuzzy logic controller uses the torque of a combustion engine, the state of charge of an energy-storage, and the rotational speed of the combustion engine as input quantities, and determines torque sharing between the combustion engine and the electric machine in accordance with pre-determined fuzzy logic rules. The approach based on fuzzy logic is however quite complex. Furthermore, finding a suitable set of fuzzy logic rules may require a significant amount of effort and experiments.